It has been established that early diagnosis of breast cancer significantly improves prognosis by enabling effective therapy and by avoiding metastatic spread of disease. To this end, breast cancer screening, primarily of post-menopausal women, has been introduced in many countries. Such screening is currently predominantly based on mammography.
The use of mammography on post-menopausal women is predicated on the observation that the menopause initiates a complex process known as involution whereby dense tissue converts gradually to fat. ‘Dense tissue’ describes the fibroglandular (glandular and connective) tissue in the breast. Volumetric breast density is defined as the volume of fibroglandular tissue divided by the volume of the breast. Dense tissue attenuates x-rays to a greater extent than fat, which is relatively transparent.
Since tumours and microcalcifications (often a precancerous sign) are also radiologically dense, they are often relatively easy to detect in the breast of patients whose breasts are primarily fatty. Indeed, it has been shown that the chance of detecting tumours and microcalcifications in a patient with a fatty breast is high, sometimes up to 98%. Thus, standard mammography is known to be effective for use in screening fatty breasts, as a lack of dense tissue does not hide cancers and there is consequently less chance of tissues being projected on to one another to give the appearance of tumour from resultant overlap.
Tomosynthesis is a near-3D form of mammography and thus tomosynthesis images are not so compromised by projection of overlapping tissues. However cancers can still be hidden in dense tissue in an individual ‘slice’, the reconstruction can blur density across slices. Tomosynthesis is consequently not ideal for very dense breasts, and some studies undertaken to demonstrate the efficacy of tomosynthesis to detect microcalcifications have been inconclusive.
The chance of detecting tumours and microcalcifications in a patient with a dense breast is around 50%, thus assessment of mammograms increasingly comprises estimation and consideration of a patient's breast density. If the breast density is over a certain value then the patient may benefit from additional imaging techniques such as ultrasound, MRI, or DBT to aid early detection of tumours. Alternatively or additionally the clinician might choose to examine the x-rays in more detail: for example, window and levelling those areas of the breast which have dense tissue in a different fashion to those areas which are fatty, or by carefully studying change over time of dense tissue.
There are a number of reasons why breast tissue may be dense: for example, a patient may not have entered the menopause (mammography has been shown to be not as effective clinically in younger or pre-menopausal women); the breast may not involute to fat for some reason; the woman might be taking hormone replacement drugs (or similar); or she might simply have lost weight. Information about change over time is clinically important: it has been reported, for example, that women whose breast density does not decrease with age are at more risk of developing breast cancer.
It is neither cost effective nor practical for every patient to undergo imaging additional to mammography not only due to the inherent cost of such extensive imaging, but also due to the numbers of false positives likely generated and, with certain additional imaging, the risk associated with extra radiation.
It is thus becoming a prerequisite for additional imaging that there exists an objective, reproducible, trans-modality, quantitative method of assessing breast density to ensure that only those women who need additional imaging receive it, due to the masking risk of dense tissue, or increased risk of cancer developing.